The invention relates generally to air pollution control technology and more specifically to the removal of particulate matter and sulfur oxides (SO.sub.x, SO.sub.2) or other acidic gases, such as HCl, from contaminated gases. The following description of the invention refers primarily to removal of SO.sub.x and particulates, since most of the experimental work by the inventors involved removal of oxides of sulfur and particulates from contaminated gases. However, because of the nature of the invention it can be utilized to remove acidic gases other than SO.sub.x. Thus, the reference to SO.sub.x and SO.sub.2 removal in this application is intended to include the removal of other acidic gases and particulates from a gas stream, and should not be construed as limiting the invention to the removal of only SO.sub.x and particulates.
In recent years, electric power generating plants and industrial boilers have been increasingly relying on high sulfur coals as a relative inexpensive fuel. This in turn results in the generation of excessive amounts of SO.sub.x. As a result, the seriousness of the "acid rain" problem has been increasing.
Additionally, coal burning creates a large amount of particulate matter. This particulate matter often remains suspended in the air for a long period of time, thus creating a serious health problem. Since these particulate materials often are, or quickly become, charged, they tend to settle on oppositely charged surfaces. Thus, the particulates cause unsightly blemishes on building, trees, curtains, etc. Additionally, these particulates can settle on machinery and electronic equipment, causing equipment malfunctions and increased wear and tear, resulting in economic loss.
Thus, there have been several efforts to remove SO.sub.x and to remove particulate matter from contaminated air. For example, in a process described by Kisters et al, in U.S. Pat. No. 4,229,411 (hereinafter referred to as Kisters et al), an aqueous solution of a neutralizing agent, such as sodium hydroxide or milk of lime is sprayed into sulfur oxides containing air. Initially, the neutralizing agents react with sulfur oxides, yielding wet salts. As the drops of water evaporate, the air is cooled and dry salts are formed. The dry salts are then removed from the gas by an electrofilter.
In both Rich (U.S. Pat. No. 4,349,359) and Fitch et al, small air borne particles are removed by charging the particles in a precharger and then subjecting the particles to an electrostatic precipitator.
Heretofore, there have been no attempts to combine SO.sub.x control and particulate control technology in a single unit. A major reason why such a combination of technologies has not been attempted has been that each of these technologies has obtained a separate status in the art. Also, it has been thought that to combine SO.sub.x and particulate control in a single apparatus or method, it was necessary to provide the spray and the precipitator systems separately, thereby adding to the complexity, size and cost of the necessary apparatus. For example, most neutralizing spray systems have been incorporated in the ductwork leading to the electrostatic precipitator (hereafter referred to at times as ESP).